EM waves

I have always been uncertain about what an EM wave consists of. I used to think it was a photon and somehow it carried properties of a wave. From reading some of the other posts on this forum, I'm realizing that I probably far off.

So what is it? Does it have mass? Electro-magnetic... I'd assume it has something to do with electrons, does it and how so?

Originally posted by Farn I have always been uncertain about what an EM wave consists of. I used to think it was a photon and somehow it carried properties of a wave. From reading some of the other posts on this forum, I'm realizing that I probably far off.

you are correct. one can think of light as either a wave or a particle, this is called duality.

Does it have mass?

light has no rest mass. which basically means it has no mass. (which is obvious, because it could not accelerate to c if it did)

Electro-magnetic... I'd assume it has something to do with electrons, does it and how so?

no, it has nothing to do with electrons. electro-magnetic was a term invented by Maxwell when he discovered light's shared properties of magetism and electricity. i think.

Originally posted by Farn I have always been uncertain about what an EM wave consists of. I used to think it was a photon and somehow it carried properties of a wave. From reading some of the other posts on this forum, I'm realizing that I probably far off.

So what is it? Does it have mass? Electro-magnetic... I'd assume it has something to do with electrons, does it and how so?

Straighten me out people!

In addition to that said, I would add that the key to understanding EM at one level is to realize two things. First, if we do an experiment to detect light particles, we get particles. If we look for waves, we get waves. The experiment determines the nature of what we observe. This all relates to Heisenberg's Uncertainty principle; and from this we get the notion that the experimenter is an inseparable part of the experiment. We determine, or we select the reality by the way that we look.

The other thing to understand is how an EM wave is sustained. By the wave model, it starts as a change, for example, in the magnetic field strength of a wire or an antenna. But a change in a magnetic field also produces a changing electric field. As one field collapses, it produces the other. The energy of the photon is shared between these two fields. So really all we have are two changing fields that create the other, and that move in a specific direction as they do so.

Finally, I would also add that this notion that light has no mass is really a little misleading. A photon has no rest mass, but a photon is never at rest. It never has to "get to the speed of light". It can be argued that the mass of a photon is just h&mu;/C^2. Perhaps this approach fails at higher levels, but I believe this is consistent with General Relativity. Objections anyone?

Edit: Quickly I add, we do normally use the language the a photon has no rest mass; in spite of my personal objections.

Correct me if I am wrong, considering the fact that light is attracted to a black hole proves that light indeed has mass?
As far as what an EM wave is. Simply put Electricity and Magnetic waves of energy propigating through space. The electricity produces the magnetic when it is expanding and the magnetic is inducing the electric when it is collapsing. for ever and ever and ever until reactance of some sort stops it.

Originally posted by sheldon Correct me if I am wrong, considering the fact that light is attracted to a black hole proves that light indeed has mass?
As far as what an EM wave is. Simply put Electricity and Magnetic waves of energy propigating through space. The electricity produces the magnetic when it is expanding and the magnetic is inducing the electric when it is collapsing. for ever and ever and ever until reactance of some sort stops it.

Light is "attracted" to mass because a source of mass large enough distorts the very fabric of spacetime. Because light travels in a straight line, it looks as if it is curving to us 3d existing humans. But really light is just following the straightest path of curve spacetime. That's way gravitation lensing happens too.

The other thing to understand is how an EM wave is sustained. By the wave model, it starts as a change, for example, in the magnetic field strength of a wire or an antenna. But a change in a magnetic field also produces a changing electric field. As one field collapses, it produces the other. The energy of the photon is shared between these two fields. So really all we have are two changing fields that create the other, and that move in a specific direction as they do so.

Ivan, you mention a 'changing electric field' and a 'changing magnetic field' in your explanation. When I think of an electric field I think of electrons when I think of a magnetic field, I think of a current of electrons. However...as its been said an EM wave doesn’t require electrons to be present for it to propagate and of course this is true because we know they cross space. So I'm still at a loss for how these changing fields take place without electrons. What am I missing?

Originally posted by neutroncount Light is "attracted" to mass because a source of mass large enough distorts the very fabric of spacetime. Because light travels in a straight line, it looks as if it is curving to us 3d existing humans. But really light is just following the straightest path of curve spacetime. That's way gravitation lensing happens too.

As I understand this [it's possible that I don't entirely], using the concept of the photon's mass yields the same result as calculating the deflection due to spacetime warpage. As stated, I believe that the concept of photon mass is consistent with GR. In the tensor, I don't think that the two approaches can be differentiated.

Originally posted by Farn Ivan, you mention a 'changing electric field' and a 'changing magnetic field' in your explanation. When I think of an electric field I think of electrons when I think of a magnetic field, I think of a current of electrons. However...as its been said an EM wave doesn’t require electrons to be present for it to propagate and of course this is true because we know they cross space. So I'm still at a loss for how these changing fields take place without electrons. What am I missing?

At the deepest levels, I am not qualified to answer this question correctly. To the extent that I can answer this I will try. Electrons have an electric field. The electron is one concept; the field is the other. It is not correct to think of the field as an extension of the electron. The field is a consequence of the internal workings of the electron - we would have to talk about quarks at this point and I'm not qualified to do so. But the key point is that the field and the particle are two different things.

Next, we get in deep quickly with Maxwell's equations and the implicit relativity theory therein. Maxwell tells us that an electric field is produced by a collapsing magnetic field. Also, as the electric field forms, it produces a magnetic field. This, at one level, is the best description of these two fields to be found. Anything else is just a less precise way of thinking about it. Also, each of these fields contains energy. The total energy of the photon could be found [in principle] by simultaneously measuring each field and then calculating and adding up the energies.

At another level, and I resist telling you this because it can easily cause confusion especially in this context, but, here we go. We do have a better explanation for the nature of these two fields. An electric field is just an exchange of photons! So, you seek to understand how the electric field manifests as a photon, and yet the photon IS the essence of the electric field. Honestly, I don't know how to explain the apparent contradiction. Perhaps someone else can help here. But it gets even worse.

Imagine that you are standing next to a wire in which current flows. As you mentioned, you could measure a magnetic field around the wire due to the flow of charge in the wire. Now, instead of a wire, imagine that we are talking about a stream of electron moving through a vacuum. Here comes the relativity part: If you could pace the flow of electrons moving through the vacuum by running alongside the stream of charge, the electrons would no longer appear to be moving. Correct? It would appear that they are motionless as compared to you. Guess what? If we have no moving charge, we don't have a magnetic field. Right? So when we started, you could measure a magnetic field as you stood next to the stream of electrons. But as soon as you start to move with the electrons, for you, the magnetic field disappears! A magnetic field is just an electric field viewed relativisticly!

Physics has the concept of fields existing in empty space (or "the vacuum"). This concept continues in modern quantum FIELD theory.

Photon are quanta (excitations) of the electromagnetiic field. Sometimes the photon is the appropriate way to analyze a given problem and sometimes the field is better.

EM is a perfectly good effective theory. At its own scale it gives accurate results, and in waveguides you can measure the sparate electric and magnetic components of the field coming down the pipe at right angles to each other.

Here's another thing. The Langrangian for quantum electrodynamics is made up of three terms added together. The first term is the lagrangian for Dirac's theory of the electron. The second term is the F[mu][nu]F[mu][nu] lagrangian for relativistic EM. The third term describes the interaction. In a free (no interaction) case, you have a separate - or no - electron and Maxwell's EM.

Originally posted by selfAdjoint Photon are quanta (excitations) of the electromagnetiic field. Sometimes the photon is the appropriate way to analyze a given problem and sometimes the field is better.

Hopefully this is not a stupid question; being a real wimp at QED it just ocurred to me : When we talk about photon exchange as the electric field, what determines the frequency of the photons? I would tend to assume that the electric field strength goes as photon frequency, but since classically the field strength depends entirely on the quantity of charge [assuming no magnetic field] present, which seems to mean that a stronger electric field has more photon exchanges but not stronger ones, I don't see what would detemine frequency of the photons. Photon enlightenment anyone?

Originally posted by neutroncount Light is "attracted" to mass because a source of mass large enough distorts the very fabric of spacetime. Because light travels in a straight line, it looks as if it is curving to us 3d existing humans. But really light is just following the straightest path of curve spacetime. That's way gravitation lensing happens too.

It’s been a little while since I first posted this, but as I read back over the responses there is one main question that pops into mind: What is a photon exchange, and how does it relate to an electric field?

Originally posted by Farn First I'd like to say thanks for the many replies!

It’s been a little while since I first posted this, but as I read back over the responses there is one main question that pops into mind: What is a photon exchange, and how does it relate to an electric field?

One simple way to think of a photon exchange is to imagine that you and I are throwing baseballs at each other. This might be considered an analogy of a repulsive electric force - such as when two like charges repel each other. If I throw the ball, I get a little push as I throw the ball towards you. If you catch a ball, you get a little push from the ball. A photon exchange is kind of like this - like throwing massless baseballs [actually quanta of electromagnetic energy] at each other thus causing us move farther and farther apart. The actual situation is much more complicated but that's the idea.

It should be understood that this idea of a photon exchange is a more sophisticated idea than what is taught at least during the beginning semesters of college physics. Don’t allow this to confuse you when you hear other explanations for the electric field. One must remember that various scientific models can exist to explain the same thing. Each model is valid to within a certain level of precision, and given that certain criteria are met.

Until you begin to study Quantum Electrodynamics, not much of this can mean much. If and when you do study such subjects, it still won't mean much! You will just be learning how to manipulate the mathematics of a model that at the deepest levels we mere mortals really can't and may never understand.

Originally posted by Ivan Seeking Finally, I would also add that this notion that light has no mass is really a little misleading. A photon has no rest mass, but a photon is never at rest. It never has to "get to the speed of light". It can be argued that the mass of a photon is just h&mu;/C^2. Perhaps this approach fails at higher levels, but I believe this is consistent with General Relativity. Objections anyone?

Photons travel at a speed which depends on the optical density of the substance through which they travel. It is only in a vacuum that photons travel at 3.0x10^8 m/s Just recently, some scientists did manage to completely freeze a beam of light within some device.

So...
Technically it is feasible to speak about the rest mass of a photon.

Originally posted by Ace-of-Spades Just recently, some scientists did manage to completely freeze a beam of light within some device.

So...
Technically it is feasible to speak about the rest mass of a photon.

I think you are misunderstanding the notion of slowing the speed of light. In the experiments to which you refer, what slows the apparent speed of light is the absorption and re-emission of photons. The average speed measured is then a function of the travel speed, coupled with the time for these absorptions and emissions. Also, the last that I checked, the slowest speed measured was about 3m/s.

Are you sure ????
Because I'm pretty sure I read an issue of New-Scientist that had an article about some guys who managed to fully freeze a beam of light. I could tell you which issue it was once I get back to university in August [?]

ps. Do you mean that light travels slower through water simply because the photons keep colliding with H2O which gets excited and releases new photons?

- I never really thought of it that way, and it makes quite a lot of sense... hmmm interesting

I am a little confused...EM waves are produced by moving (accelerating) electrons...Light is an EM wave, so that means light is just a visible form of EM wave which is produced by some form of acclerating electrons? Am I correct?

Originally posted by Ace-of-Spades Are you sure ????
Because I'm pretty sure I read an issue of New-Scientist that had an article about some guys who managed to fully freeze a beam of light. I could tell you which issue it was once I get back to university in August [?]

This is my understanding. Also, the equations for light do not allow this to happen. The nature of the photon is one of constant velocity - while a photon. It can only be absorbed and the energy conserved [stored], typically as an exited electron state, until re-emitted. It can also be absorbed and converted to heat energy.

ps. Do you mean that light travels slower through water simply because the photons keep colliding with H2O which gets excited and releases new photons?

- I never really thought of it that way, and it makes quite a lot of sense... hmmm interesting

I believe this is a correct way to think of it. Yes it is all very interesting!